Home Solar System Size Estimator
Calculate exactly how many kilowatts of solar panels you need and how many panels to install to cover your home's electricity usage — based on your monthly kWh consumption and your region's sun hours.
About this calculator
This estimator works out the solar array size your home needs to offset a chosen percentage of its electricity consumption. It takes your monthly kWh usage from your utility bill, scales it to an annual figure, then divides by the number of productive kWh a 1 kW system would generate in your region (peak sun hours × 365 × system efficiency). The result is the raw DC nameplate capacity in kilowatts, rounded up to the nearest half-kilowatt for realistic installer quotes. It then divides that capacity by your chosen panel wattage to give you a panel count, and multiplies by the standard panel footprint (~17.6 sq ft for a 400 W panel) to estimate the roof area required.
Field explanations
- Monthly electricity usage (kWh)
- Your average monthly electricity consumption in kilowatt-hours. Find this number on your utility bill — it's usually labeled "kWh used" or "energy consumed." Use a 12-month average if possible, since summer cooling and winter heating can cause large swings. The US residential average is about 900 kWh/month.
- Coverage target
- What percentage of your annual electricity use you want the solar system to offset. 100% means the system is sized to produce as much energy as you consume over the year. Choose 80% for a smaller upfront cost or 120% if you plan to add an EV, a heat pump, or a battery storage system in the future.
- Peak sun hours per day
- The number of hours per day when sunlight intensity averages 1,000 W/m² at your location. This is not total daylight hours — it's a measure of solar irradiance intensity. A location can have 12 hours of daylight but only 4.5 peak sun hours due to clouds, haze, and low sun angles in the morning and evening. Use the regional presets as a starting point, or look up your exact address using NREL's PVWatts calculator.
- Panel wattage
- The rated output of each solar panel in watts under standard test conditions. Modern residential panels range from 350 W to 500 W. Higher-wattage panels cost more per panel but require fewer panels and less roof space for the same system size. 400 W is a common default for current residential installations.
- System efficiency (derate)
- A multiplier (as a percentage) that accounts for all real-world losses between the DC output of the panels and the AC power delivered to your home. Losses come from the inverter (~96–98%), wiring resistance, temperature (panels lose output on hot days), dust and soiling, and minor shading. The NREL PVWatts default is 78%; a modern microinverter system in a clean environment can reach 85%.